In known vehicle speed control systems, typically referred to as cruise control systems, the vehicle speed is maintained on-road once set by the user without further intervention by the user so as to improve the driving experience for the user by reducing workload.
With typical cruise control systems, the user selects a speed at which the vehicle is to be maintained, referred to as a set-speed, and the vehicle is maintained at a target speed that is set equal to the set-speed for as long as the user does not apply a brake or, in the case of a vehicle having a manual transmission, depress a clutch pedal. The cruise control system takes its speed signal from a driveshaft speed sensor or wheel speed sensors. When the brake or the clutch is depressed, the cruise control system is disabled so that the user can override the cruise control system to change the vehicle speed without resistance from the system. If the user depresses the accelerator pedal by a sufficient amount the vehicle speed will increase, but once the user removes his foot from the accelerator pedal the vehicle reverts to the pre-set cruise speed (set-speed) by coasting.
Such systems are usually operable only above a certain speed, typically around 15-20 kph, and are ideal in circumstances in which the vehicle is travelling in steady traffic conditions, and particularly on highways or motorways. In congested traffic conditions, however, where vehicle speed tends to vary widely, cruise control systems are ineffective, and especially where the systems are inoperable because of a minimum speed requirement. A minimum speed requirement is often imposed on cruise control systems so as to reduce the likelihood of low speed collision, for example when parking. Such systems are therefore ineffective in certain driving conditions (e.g. low speed) and are set to be automatically disabled in circumstances in which a user may not consider it to be desirable to do so.
More sophisticated cruise control systems are integrated into the engine management system and may include an adaptive functionality which takes into account the distance to the vehicle in front using a radar-based system. For example, the vehicle may be provided with a forward-looking radar detection system so that the speed and distance of the vehicle in front is detected and a safe following speed and distance is maintained automatically without the need for user input. If the lead vehicle slows down, or another object is detected by the radar detection system, the system sends a signal to the engine or the braking system to slow the vehicle down accordingly, to maintain a safe following distance.
Known cruise control systems also cancel in the event that a wheel slip event is detected requiring intervention by a traction control system (TC system or TCS) or stability control system (SCS). Accordingly, they are not well suited to maintaining vehicle progress when driving in off road conditions where such events may be relatively common. Known TC systems are arranged to intervene and cause application of brake force to reduce wheel speed if the speed of any driven wheel (being a wheel driven by the powertrain, i.e. a driving wheel) exceeds a measured value of vehicle speed, referred to as a vehicle reference speed, by more than a prescribed threshold value (such as 5 kph). The vehicle reference speed may also be used by one or more other vehicle systems that require knowledge of instant vehicle speed.
The present applicant has developed a speed control system suitable for use in off-road driving conditions. The speed control system is configured to allow driving at relatively low speeds, such as speeds in the range from 2 to 30 kph at least, and is configured not to terminate speed control if a wheel slip event is detected requiring intervention by a TC system.
Known speed control systems typically employ a closed loop feedback control arrangement to maintain vehicle reference speed substantially equal to the speed control system target speed. In known cruise control systems, for example, the feedback control arrangement generates a powertrain torque demand signal to cause the value of vehicle reference speed to remain substantially equal to a cruise control target speed.
It is also known to provide a control system for a motor vehicle for controlling one or more vehicle subsystems. U.S. Pat. No. 7,349,776 discloses a vehicle control system comprising a plurality of subsystem controllers including an engine management system, a transmission controller, a steering controller, a brakes controller and a suspension controller. The subsystem controllers are each operable in a plurality of subsystem function or configuration modes. The subsystem controllers are connected to a vehicle mode controller which controls the subsystem controllers to assume a required function mode so as to provide a number of driving modes for the vehicle. Each of the driving modes corresponds to a particular driving condition or set of driving conditions, and in each mode each of the sub-systems is set to the function mode most appropriate to those conditions. Such conditions are linked to types of terrain over which the vehicle may be driven such as grass/gravel/snow, mud and ruts, rock crawl, sand and a highway mode known as ‘special programs off’ (SPO). The vehicle mode controller may be referred to as a Terrain Response (TR)® System or controller. The driving modes may also be referred to as terrain modes, terrain response modes, or control modes.